| Distributed generation based on renewable energy can relieve the shortage of energy and environmental pollution problems all over the world, consequently promote the power industry. But there are also some cooperant problems between large power grids and distributed generators. To exert the effect and benefit of distributed generator, and simplify the connection of distributed generator, micro-grid operation appeared and be highlighted for special attention.The existence of generators in micro-grid power systems makes the power flow changed from unidirectional to multidirectional. As a whole system of power source and load, micro grid has two operation modes: connection and islanding. To reduce the disturbance of large power system and maintain the stability of micro-grid, how and when to switch micro grid operation between connection and islanding becomes a key issue. This paper is about the out-of-step disconnection of micro-network. The out-of-step disconnection is the last ditch to prevent power system breakdown in traditional large power system, so it must operate based on out-of-step sure, or there would be a lot of load lost for misoperation. But the influence of misoperation of micro-grid is not as important as that in large power system. Out-of step is only one of the disconnection conditions of micro-grid. The requirement different of out-of-step operation between HV power system and micro-grid makes the traditional out-of-step judgment methods not entirely suitable in micro-grid.This paper analyzes some parameters'characteristic and variation when swing or out-of-step occurs, and their applicability in micro-grid out-of-step disconnection. These parameters contain impedance, P and Q, impedance angle, power angle and U cos(?). Based on these analyses, this paper gives an out-of-step algorithm using fuzzy logic. This algorithm combines voltage, frequency, power angle and their changing rate, and algorithm tests show that this new algorithm is more time saving and reliable.
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